An increased attention is recently paid to a non-impact printing technique in view that a generation of a sound at a recording time is significantly as small as can be ignored.
In particular, an ink jet method which can print on a plain paper at a high speed by using a comparatively simple mechanism is a significantly useful printing technique, various kinds of techniques have been suggested, and a technique which is suitable for a high speed printing, a high resolution and a full color printing has been eagerly going to be researched. Among them, there is representatively a multi nozzle type which prints a plurality of dots in parallel, for example, a bubble jet method which discharges an ink drop due to a pressure of a steam generated by a heat of a heat generating body and a piezoelectric method which discharges an ink drop due to a mechanical pressure pulse generated by a piezoelectric element.
However, there is a problem that the conventional ink jet printer is not suitable for improving a resolution. That is, in the bubble jet method which employs the pressure of the steam, it is hard to generate an ink drop having a diameter smaller than 20 .mu.m, and in the piezoelectric method which employs the pressure generated by the piezoelectric element, it is hard to produce a head having a high resolution due to a problem on a processing technology since the recording head has a complex structure.
Further, the ink employed in the conventional ink jet method has a lot of technical problems.
In this case, as characteristics required in the ink for the ink jet, firstly, an even image having a high density without a bleeding and a photographic fog can be obtained on a paper, secondly, a weather resistance of the image is good, thirdly, a drying property of the ink is good on the paper, and fourthly, no clogging is generated and a discharge stability and a discharge response are excellent.
In order to obtain these characteristics, there has been proposed various kinds of electrostatic ink jet methods which apply a voltage on an electrode array formed in a thin film and employ an electrostatic force so as to discharge the ink.
Here, a description will be given of a technique disclosed in Japanese Patent National Publication of translated version 7-502218, as the conventional electrostatic type ink jet method.
The conventional electrostatic ink jet technique in accordance with the publication is structured such as to apply a voltage having the same polarity as that of a charged color material particle to an electrode at a front end of a slit so as to form an aggregate of the color material particles and discharge the aggregate of the color material particles from a front end of a printing electrode.
Then, in accordance with the technique, since the color material particles are discharged in a cohered state, an ink having a little solvent is formed on the paper as a dot, so that a printing having a high density and a less bleeding can be realized. Further, since the solvent is reduced, the ink is quickly dried on the printing medium. Further, since a pigment is employed for the color material particles of the ink as is different from the other ink jet methods which employ a dye ink, it is possible to obtain an image having an improved weather resistance.
Further, in the conventional electrostatic type ink jet technique, since the printing head is structured such as to be formed in a slit shape which does not require independent nozzles at every dots, it is effective for preventing and repairing a clogging which causes a great problem for putting the ink jet head to a practical use, so that the discharge stability is always good and a reliability can be maintained.
Further, the conventional electrostatic ink jet technique can easily form a printing dot having a diameter about 10 to 20 .mu.m in accordance with a length of a printing signal pulse, and can also form a large printing dot having a diameter equal to or more than 100 .mu.m. Accordingly, since it is possible to achieve a multi value area gradation as well as a high resolution, it can be said to be an ink jet method which is most suitable for obtaining a high resolution and a full color.
Hereinafter, a description will be given of a structure and an ink discharge process of an electrostatic type ink jet head, and a characteristic of an ink for the electrostatic type ink jet. In this case, hereinafter, the ink for the electrostatic type ink jet is simply referred to as an ink.
FIG. 1 is a schematic view which shows a structure of an electrostatic type ink jet head, FIG. 2 is a schematic view which shows a structure of an inner portion of the electrostatic type ink jet head shown in FIG. 1, FIG. 3 is a side cross sectional view of the electrostatic type ink jet head shown in FIG. 1, and FIGS. 4 to 7 are schematic views which show a discharge motion of an ink performed by the electrostatic type ink jet head shown in FIG. 1 in a subsequent manner.
As illustrated, a printing head has a lower casing 7 and an upper casing 8 which are bonded to each other so as to be integrally formed. A slit hole 2 is formed at a front end of the printing head, and a plurality of printing electrodes 1 which are driven by a printing electrode driver so as to discharge an ink drop are provided in such a manner as to extend inward from the slit hole 2.
A front end of the printing electrode 1 s formed in a convex shape and is placed so as o protrude from the slit hole 2 corresponding to discharge hole at a degree of 50 to 200 .mu.m so that the printing electrode 1 can concentrate an electric field to the ink existing near the front end of the printing electrode 1 and a stable discharge can be performed by stably forming a meniscus.
Further, the printing electrode 1 is constituted by a lead wire 3 for applying a signal voltage, and a tab wiring substrate 4 which is integrally formed with a pad (not shown) for being electrically connected to the printing electrode driver.
An ink tank 6 in which an ink is charged is formed within the printing head by an opposing space between the lower casing 7 and the upper casing 8 so as to be communicated with the slit hole 2. A migration electrode 5 is provided on an inner surface of the ink tank 6 for applying an electrophoresis in a direction of the slit hole 2 to the color material particle within the ink tank 6 and increasing a density of the color material near the slit hole 2.
Here, the tab wiring substrate 4 and the migration electrode 5 mentioned above are bonded to the lower casing 7. Further, the pad is bonded to an FPC wiring substrate (not shown). In this case, a matrix circuit, a driver IC and the like are mounted on the FPC wiring substrate.
As shown in FIG. 4, in this electrostatic type ink jet head, in a state that the ink is charged within the ink tank 6 and within a slit-like ink flow passage, the ink forms a meniscus 9 within a periphery of the slit hole 2 due to a surface tension. Then, since a back pressure about .+-.100 Pa is applied to the ink within the ink tank 6, the meniscus 9 is formed in a state of gently protruding from the ink discharge hole.
In this case, a printing paper 10 corresponding to a printing medium for the ink is arranged in a direction of discharging the ink, and an opposing electrode 11 which discharges the ink toward the printing paper 10 due to an electrostatic force is arranged on a back surface of the printing paper 10. In this case, in the case that a color material particle 12 is charged in a positive potential, a negative voltage about -1 kV is applied to the opposing electrode 11 at a time of printing, whereby a potential difference with respect to the printing electrode 1 is controlled. Further, as a voltage applied to the printing electrode 1, a positive voltage in a range between 200 and 1000 V is used. However, a printing voltage has no upper limit, and is generally determined in accordance with a specification of a usable driver IC. Since the driver IC tends to be expensive as the drive voltage becomes higher, in the case of using an electrostatic ink jet printer as a wide use printer for an office and a personal use, an inexpensive printer can be provided when a signal voltage is made lower.
A description will be given of an ink discharge operation by the electrostatic type ink jet head having the structure mentioned above with reference to FIGS. 4 to 7.
FIG. 4 shows a state that a printing is not performed. A voltage is applied to the opposing electrode 11 and a voltage is not applied to the printing electrode 1. In this state, the meniscus 9 is formed in the slit hole 2 in such a manner as to gently protrude along a shape of a front end of the printing electrode. Then, at a time of printing, the charged color material particle 12 within the ink tank 6 is performed an electrophoresis in a direction of the slit hole 2 corresponding to the front end of the printing electrode 1 when a voltage equal to or more than the printing voltage is applied to the migration electrode 5 (refer to FIGS. 2 and 3). At this time, when a signal pulse voltage is applied to the printing electrode 1 by the printing electrode driver, an electric field is concentrated to the front end of the printing electrode 1 and the color material particle 12 is cohered, so that the meniscus 9 constituted by the color material particle 12 having a high density and a little amount of solvent starts deforming.
Then, as shown in FIG. 5, the meniscus 9 is formed in an ink drop shape due to the electrostatic force and grows toward the opposing electrode 11.
Then, finally, as shown in FIG. 6, an ink drop 13 is separated from the meniscus 9 and is discharged in a state of a liquid drop.
When the signal pulse voltage is turned off, the discharged ink drop is attached to the printing paper 10 arranged between the printing electrode 1 and the opposing electrode 11, as shown in FIG. 7. Finally, the ink is heated and fixed to the printing paper 10 by a heater (not shown), thereby printing on the printing paper 10.
When the discharge of a desired ink drop 13 is finished, the color material particle 11 within the ink tank 6 moves in a direction of the slit hole 2, whereby the color material particle 11 is supplied near the printing electrode 1 and as shown in FIG. 7, the meniscus 9 of the printing electrode 1 is formed in the slit hole 2 so as to gently protrude along the front end shape of the printing electrode 1 in the same manner as that of an initial state before discharging the ink.
Thereafter, the operation of discharging the ink mentioned above is repeated, and the printing is continuously performed.
The ink discharge in the electrostatic type ink jet head mentioned above is characterized by increasing a density of the color material particle 12 near the discharge position so as to take out the ink having a high color material particle density. Then, since the ink which is taken out includes the color material particle 12 having a high density and the same polarity, the ink is separated due to an electrostatic repulsion between the color material particles 12 so as to form fine ink drops and is discharged toward the opposing electrode 11. The electrostatic type ink jet heat mentioned above is characterized by injecting the color material particle 12 in a state of making the density of the color material particle 12 in the ink higher than that of the original ink, thereby improving a selecting property of generating the ink drop from each of the printing electrodes 1 by utilizing the difference in density of the color material particle.
Next, a description will be given of a characteristic of a conventional ink employed for the electrostatic type ink jet head.
For example, in Japanese Patent National Publication of translated version 8-512069, there is disclosed an ink composition for an ink jet which contains a solvent having an electric resistance equal to or more than 10.sup.9 .OMEGA.cm, a marking particle being insoluble and capable of being electrically charged, a particle charging agent, and is structured such as to be indispensable for the ink used in the conventional electrostatic type ink jet head.
That is, the conventional ink is characterized by using the ink in which the color material particle charged in the solvent having a high volume resistivity is dispersed. The details thereof is not disclosed in the publication, however, generally, for example, there is employed an isoparaffin hydro carbon, a silicone oil or the like is employed for the solvent, and an ink constituted by a color material structured such as to contain a color material particle such as a carbon black or the like on a binder made of a resin or a wax or a surface, a dispersing agent, an electric charge controlling agent and the like.
Here, a dielectric solvent having a high electric resistivity is required for the solvent. This is because the electric field applied to the ink can reach the color material particle via the solvent by employing the dielectric solvent. Then, in order to perform an electrophoresis the color material particle, it is necessary to charge the color material particle itself. Since a fixed liquid drop amount of color material particle is discharged from the ink by utilizing the electrostatic repulsion, a static amount of the charge is required.
Further, the ink contains an addition agent. That is, for example, adding a dispersion assisting agent to a printing liquid, the color material particle can stably disperse in the solvent without being cohered.
Further, by adding the electric charge controlling agent to the ink, it is possible to improve an electric charge characteristic of the color material particle.
As mentioned above, the ink can be obtained by mixing the resin, the coloring agent and the particle electric charging agent and dispersing the color material particle obtained by pulverizing to a desired diameter of the particle, in the solvent together with a little amount of dispersion assisting agent.
Further, in Japanese Patent Unexamined Publication No. 9-193389, there is disclosed an ink adjusted so as to have an electric resistivity equal to or more than 10.sup.8 .OMEGA.cm by dispersing a developing particle having a predetermined polarity, a dielectric solvent having an electric resistivity equal to or more than 10.sup.10 .OMEGA.cm, an ink constituted by a developing particle having a .zeta. potential equal to or more than 60 mV, an average diameter of the particle between 0.01 and 5 .mu.m and an electric resistivity equal to or more than 10.sup.8 .OMEGA.cm, and the like. In the ink mentioned above, since no bleeding is generated on various kinds of printing medium, it is suitable for a high quality printing.
Further, in Japanese Patent Unexamined Publication No. 8-291267, there is disclosed an ink in which an amount of specific image (Q/M) of a charged particle is between 10 and 1000 .mu.C/g and an electric resistance of an ink composition is equal to or more than 10.sup.10 .OMEGA.cm. In the ink mentioned above, since the ink is discharged by a low applied voltage, it is suitable for a printing having a high image density, a high contrast and a high resolution.
However, in the conventional ink mentioned above, there is the following problems at a time of cohering the color material particles so as to discharge.
That is, since the electrostatic type ink jet is structured such as to collect the color material particles having a high density to the front end of the printing electrode due to an electrophoresis, it takes a comparatively long time to perform an electrophoresis the color material particles so as to reach near the printing electrode. Accordingly, in the case of printing in a high printing frequency, there is a case that the color material particles are insufficiently supplied to the printing electrode, thereby being discharged in a low density of the color material or not being discharged. Then, as a result thereof, a printing density becomes light and a dispersion of a diameter of the printed dot becomes large, thereby deteriorating a printing quality.
However, the conventional ink mentioned above defines a material value of the ink in view of a high quality image. That is, since there is no description concerning the material value of the ink which corresponds to a factor of the printing frequency, it is not said that the ink takes an improvement of the printing frequency into consideration.
Then, in order to realize the ink which can obtain a high quality image even by printing at the high printing frequency, it is necessary to define the material value of the ink in view of the printing frequency, for example, in view of increasing a speed of the electrophoresis of the color material particle so as to supply the color material particle near the printing electrode at a high speed and in a stable manner.